Managing the gas–liquid interface within gas‐diffusion electrodes (GDEs) is key to maintaining high product selectivities in carbon dioxide electroreduction. By screening silver‐catalyzed GDEs over a ...range of applied current densities, an inverse correlation was observed between carbon monoxide selectivity and the electrochemical double‐layer capacitance, a proxy for wetted electrode area. Plotting current‐dependent performance as a function of cumulative charge led to data collapse onto a single sigmoidal curve indicating that the passage of faradaic current accelerates flooding. It was hypothesized that high cathode alkalinity, driven by both initial electrolyte conditions and cathode half‐reactions, promotes carbonate formation and precipitation which, in turn, facilitates electrolyte permeation. This mechanism was reinforced by the observations that post‐test GDEs retain less hydrophobicity than pristine materials and that water‐rinsing and drying electrodes temporarily recovers peak selectivity. This knowledge offers an opportunity to design electrodes with greater carbonation tolerance to improve device longevity.
A matter of time: Gas‐diffusion electrodes (GDEs) for carbon dioxide electroreduction are screened over a range of applied current densities, and an inverse correlation is observed between carbon monoxide selectivity and the electrochemical double‐layer capacitance, a proxy for wetted electrode area. This knowledge offers an opportunity to design electrodes with greater carbonation tolerance to improve device longevity.
Sluggish vanadium reaction rates on the porous carbon electrodes typically used in redox flow batteries have prompted research into pretreatment strategies, most notably thermal oxidation, to improve ...performance. While effective, these approaches have nuanced and complex effects on electrode characteristics hampering the development of explicit structure–function relations that enable quantitative correlation between specific properties and overall electrochemical performance. Here, we seek to resolve these relationships through rigorous analysis of thermally pretreated SGL 29AA carbon paper electrodes using a suite of electrochemical, microscopic, and spectroscopic techniques and culminating in full cell testing. We systematically vary pretreatment temperature, from 400 to 500 °C, while holding pretreatment time constant at 30 h, and evaluate changes in the physical, chemical, and electrochemical properties of the electrodes. We find that several different parameters contribute to observed performance, including hydrophilicity, microstructure, electrochemical surface area, and surface chemistry, and it is important to note that not all of these properties improve with increasing pretreatment temperature. Consequently, while the best overall performance is achieved with a 475 °C pretreatment, this enhancement is achieved from a balance, rather than a maximization, of critical properties. A deeper understanding of the role each property plays in battery performance is the first step toward developing targeted pretreatment strategies that may enable transformative performance improvements.
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•A topology optimization framework is developed to design efficient flow fields in PEM fuel cells.•Topology-optimized designs yield a significant increase in the output power compared ...to conventional layouts.•The inlet pressure has large influence on both the optimized layouts and their performance.•Specific design features allow a more homogeneous current density distribution to be obtained.
This paper presents a thorough and affordable numerical framework to design the flow field of Proton Exchange Membrane Fuel Cells using topology optimization. No assumption is made about the layout of the channels, which freely evolves along the optimization process, resulting in non-trivial optimized geometries. The optimization problem is formulated to maximize both the power generation and homogeneity of current density distribution, in the spirit of reduced costs and increased durability. The evolution of the flow field geometry is computed with a gradient-based optimizer with gradients of the objective and constraints computed through the discrete adjoint method. At each optimization iteration, the incompressible Navier-Stokes, advection-diffusion, and Butler-Volmer equations are solved with a 2D finite element model that predicts the flow in the channels, transport of chemical species and electrochemical reactions. The 3D transport effects are accounted for in the 2D model through an original depth-averaging procedure. The results of the 2D prediction are verified against a full 3D model calibrated using numerical and experimental results. The developed design framework can be used to identify flow field layouts that outperform current industrial solutions, catch design trends and provide guidelines to technology practitioners. The topology-optimized designs yield significant power generation enhancements, an improved reactant distribution and a reduced pressure drop as compared to conventional flow fields. Increasing the inlet pressure leads to more and more intricate configurations with complex topologies and highly tortuous channels. However, the cell performance is found to be more sensitive to the topology of the flow distributor at low rather than at high inlet pressures. Considering a measure of the current density homogeneity in the optimization objective allows the identification of layouts in which the gas channels concentrate close to the outlets rather than close to the inlet. These design features slightly affect the amount of power generated, suggesting a viable route for future technological development.
Porous carbonaceous electrodes are performance‐defining components in redox flow batteries (RFBs), where their properties impact the efficiency, cost, and durability of the system. The overarching ...challenge is to simultaneously fulfill multiple seemingly contradictory requirements—i.e., high surface area, low pressure drop, and facile mass transport—without sacrificing scalability or manufacturability. Here, non‐solvent induced phase separation (NIPS) is proposed as a versatile method to synthesize tunable porous structures suitable for use as RFB electrodes. The variation of the relative concentration of scaffold‐forming polyacrylonitrile to pore‐forming poly(vinylpyrrolidone) is demonstrated to result in electrodes with distinct microstructure and porosity. Tomographic microscopy, porosimetry, and spectroscopy are used to characterize the 3D structure and surface chemistry. Flow cell studies with two common redox species (i.e., all‐vanadium and Fe2+/3+) reveal that the novel electrodes can outperform traditional carbon fiber electrodes. It is posited that the bimodal porous structure, with interconnected large (>50 µm) macrovoids in the through‐plane direction and smaller (<5 µm) pores throughout, provides a favorable balance between offsetting traits. Although nascent, the NIPS synthesis approach has the potential to serve as a technology platform for the development of porous electrodes specifically designed to enable electrochemical flow technologies.
Non‐solvent induced phase separation is introduced as a versatile and facile method to synthesize redox flow battery electrodes with hierarchical microstructures unachievable via conventional manufacturing approaches. Flow cell studies reveal that the novel materials can outperform conventional carbon fiber electrodes owing to better mass transfer and higher surface area, demonstrating that this method may enable new classes of porous electrodes tailored for particular electrochemical flow technologies.
A novel method to produce gas diffusion layers with patterned wettability for fuel cells is presented. The local irradiation and subsequent grafting permits full design flexibility and wettability ...tuning, while modifying throughout the whole material thickness. These water highways have improved operando performance due to an optimized water management inside the cells.
Aqueous organic redox flow batteries are promising for low-cost and large-scale energy storage, but the redox-active molecules they rely on degrade prematurely. Now, a facile electrochemical protocol ...to regenerate them has been proposed, resulting in extended battery lifetimes.
The surface properties of porous carbonaceous electrodes govern the performance, durability, and ultimately the cost of redox flow batteries (RFBs). State-of-the-art carbon fiber-based electrode ...interfaces suffer from limited kinetic activity and incomplete wettability, fundamentally limiting the performance. Surface treatments for electrodes such as thermal and acid activation are a common practice to make them more suitable for aqueous RFBs; however, these treatments offer limited control over the desired functional properties. Here, we propose, for the first time, electrografting as a facile, rapid, and versatile technique to enable task-specific functionalization of porous carbonaceous electrodes for use in RFBs. Electrografting allows covalent attachment of organic molecules on conductive substrates upon application of an electrochemical driving force, and the vast library of available organic molecules can unlock a broad range of desired functional properties. To showcase the potential of electrografting for RFBs, we elect to investigate taurine, an amine with a highly hydrophilic sulfonic acid tail. Oxidative electrografting with cyclic voltammetry allows covalent attachment of taurine through the amine group to the fiber surface, resulting in taurine-functionalized carbon cloth electrodes. In situ polarization and impedance spectroscopy in single-electrolyte flow cells reveal that taurine-treated cloth electrodes result in 40% lower charge transfer and 25% lower mass transfer resistances than off-the-shelf cloth electrodes. We find that taurine-treated electrode interfaces promote faster Fe3+ reduction reaction kinetics as the electrochemical surface area normalized current densities are 2-fold and 4-fold higher than oxidized and untreated glassy carbon surfaces, respectively. Improved mass transfer of taurine-treated electrodes is attributed to their superior wettability, as revealed by operando neutron radiography within a flow cell setup. Through demonstrating promising results for aqueous systems with the model molecule taurine, this work aims to bring forth electrografting as a facile technique to tailor electrode surfaces for other RFB chemistries and electrochemical technologies.
The electrode thickness is a critical design parameter to engineer high performance redox flow cells by impacting the available surface area for reactions, current and potential distributions, and ...required pumping power. To date, redox flow cell assemblies employ repurposed off‐the‐shelf fibrous electrodes which feature a broad range of thicknesses. However, comprehensive guidelines to select the optimal electrode thickness for a given reactor architecture remain elusive. Here, we investigate the effect of the electrode thickness in the range of 200–1100 μm on the cell performance by stacking electrode layers in four different flow cell configurations – Freudenberg paper and ELAT cloth electrodes combined with flow‐through and interdigitated flow fields. We employ a suite of polarization, electrochemical impedance spectroscopy and pressure drop measurements together with pore network modeling simulations to correlate the electrode thickness for various reactor designs to the electrochemical and hydraulic performance. We find that thicker electrodes (420 μm paper electrodes and 812 μm cloth electrodes) are beneficial in combination with flow‐through flow fields, whereas when using interdigitated flow fields, thinner electrodes (210 μm paper electrodes and 406 μm cloth electrodes) result in a better current density and pressure drop trade‐off. We hope our findings will aid researchers and technology practitioners in designing their electrochemical flow cells under convective operation.
The effect of the electrode thickness on the electrochemical and hydraulic performance of redox flow cells is investigated. Correlations are elucidated between the electrode thickness, electrode microstructure and flow field geometry, highlighting the need to design porous electrodes for specific reactor architectures and operating conditions to enable high performance redox flow batteries.
Electrode‐assisted techniques are well suited for the separation of ions from solutions with reduced energy and chemical consumption. This emerging platform can benefit greatly from the convection ...enhanced and zero‐gap reactor designs unlocked by macroporous electrodes, but immobilizing ion‐selective layers on such complex 3D architectures is challenging. Electropolymerization of conductive polymers is proposed as a coating methodology to fabricate highly conformal coatings with electrochemically switchable ion‐exchange functionality. To demonstrate this, the synthetic parameters, resulting morphology, and ionic separation performance of poly(3,4‐ethylenedioxythiophene) (PEDOT) on commercially available carbon paper electrodes are studied. Electropolymerization of PEDOT in organic solvents results in rougher morphologies with high sensitivity to electrochemical protocols employed. When polymerized in aqueous solutions of poly(4‐styrenesulfonate) (PSS−), the resulting PEDOT/PSS blend polymer forms smooth coatings with a controllable thickness down to 0.1 µm. With appropriate voltage bias, PEDOT/PSS coated electrodes can take up and release Ni2+ in the presence of excess Na+. Increasing the coating thickness decreases the adsorption capacity due to mass transfer limitations, and the maximum adsorption capacity for Ni2+ is reached for the thinnest coatings at 228 mg g−1. Through a systematic study of PEDOT/PSS coated carbon paper, electropolymerization is identified to be a promising avenue for porous electrode functionalization.
PEDOT is electropolymerized on macroporous carbon paper substrates. The choice of counterions and solvents influences the morphology of the final coating. PEDOT/PSS can be electropolymerized conformally on the fibers of the carbon paper, with a tailored thickness down to 0.1 µm. The separation of Ni2+ from Na+ is achieved by electrochemically switched ion exchange (ESIX) on PEDOT/PSS coated porous carbon paper.
Asymmetric separators with polysulfide barrier properties consisting of porous polypropylene grafted with styrenesulfonate (PP-g-PLiSS) were characterized in lithium–sulfur cells to assess their ...practical applicability. Galvanostatic cycling at different C-rates with and without an electrolyte additive and cyclic voltammetry were used to probe the electrochemical performance of the cells with the PP-g-PLiSS separators and to compare it with the performance of the cells utilizing state-of-the-art separator, Celgard 2400. Overall, it was found that regardless of the applied cycling rate, the use of the grafted separators greatly enhances the Coulombic efficiency of the cell. An appropriate Li-exchange-site (−SO3 –) concentration at and near the surface of the separator was found to be essential to effectively suppress the polysulfide shuttle without sacrificing the Li-ion mobility through the separator and to improve the practical specific charge of the cell.
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